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  软件设计原则
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  <p>介绍一下软件常用的设计原则</p>
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<h1 id="设计原则"><a href="#设计原则" class="headerlink" title="设计原则"></a>设计原则</h1><h2 id="开闭原则"><a href="#开闭原则" class="headerlink" title="开闭原则"></a>开闭原则</h2><p>开闭原则(Open-Closed Principle, OCP)是指一个软件实体如类、模块和函数应该对扩展开放、对修改关闭。所谓的开闭，也正是对扩展和修改两个行为的一个原则。强调的是用抽象构建框架，用实现扩展细节，可以提高软件系统的可复用性及可维护性，添加一个新的功能应该是，在已有代码基础上扩展代码（新增模块、类、方法等），而非修改已有代码（修改模块、类、方法等）。开闭原则，是面向对象设计中最基础的设计原则，它指导我们如何建立稳定灵活的系统，例如:我们版本更新，尽可能不修改源代码，但是可以增加新功能。</p>
<h2 id="依赖倒置原则"><a href="#依赖倒置原则" class="headerlink" title="依赖倒置原则"></a>依赖倒置原则</h2><p>依赖倒置原则(Dependence Inversion Principle,DIP)是指设计代码结构时，高层模块不应该依赖底层模块，二者都应该依赖其抽象。抽象不应该依赖细节，细节应该依赖抽象。通过依赖倒置，可以 减少类与类之间的耦合性，提高系统的稳定性，提高代码的可读性和可维护性，并能够降低修改程序所造成的风险。</p>
<h2 id="单一职责原则"><a href="#单一职责原则" class="headerlink" title="单一职责原则"></a>单一职责原则</h2><p>单一职责(Simple Responsibility Pinciple，SRP)是指不要存在多于一个导致类变更的原因。假设我们有一个 Class 负责两个职责，一旦发生需求变更，修改其中一个职责的逻辑代码，有可能会导致另一个职责的功能发生故障。这样一来，这个 Class 存在两个导致类变更的原因。如何解决这个问题呢? 我们就要给两个职责分别用两个 Class 来实现，进行解耦，后期需求变更维护互不影响。这样的设计， 可以降低类的复杂度，提高类的可读性，提高系统的可维护性，降低变更引起的风险。总体来说就是一 个 Class/Interface/Method 只负责一项职责。</p>
<h2 id="接口隔离原则"><a href="#接口隔离原则" class="headerlink" title="接口隔离原则"></a>接口隔离原则</h2><p>接口隔离原则(Interface Segregation Principle, ISP)是指用多个专门的接口，而不使用单一的总接口，客户端不应该依赖它不需要的接口。</p>
<p>这个原则指导我们在设计接口时应当注意一下几点:</p>
<ol>
<li><p>一个类对一类的依赖应该建立在最小的接口之上。</p>
</li>
<li><p>建立单一接口，不要建立庞大臃肿的接口。</p>
</li>
<li><p>尽量细化接口，接口中的方法尽量少(不是越少越好，一定要适度)。 </p>
</li>
</ol>
<p>接口隔离原则符合我们常说的高内聚低耦合的设计思想，从而使得类具有很好的可读性、可扩展性和可维护性。我们在设计接口的时候，要多花时间去思考，要考虑业务模型，包括以后有可能发生变更的地方还要做一些预判。所以，对于抽象，对业务模型的理解是非常重要的。</p>
<h2 id="迪米特法则"><a href="#迪米特法则" class="headerlink" title="迪米特法则"></a>迪米特法则</h2><p>迪米特原则(Law of Demeter LoD)是指一个对象应该对其他对象保持最少的了解，又叫最少知道原则(Least Knowledge Principle,LKP)，尽量降低类与类之间的耦合。迪米特原则主要强调只和朋友交流，不和陌生人说话。出现在成员变量、方法的输入、输出参数中的类都可以称之为成员朋友类， 而出现在方法体内部的类不属于朋友类。</p>
<h2 id="里氏替换原则"><a href="#里氏替换原则" class="headerlink" title="里氏替换原则"></a>里氏替换原则</h2><p>里氏替换原则(Liskov Substitution Principle,LSP)是指如果对每一个类型为 T1 的对象 o1,都有类型为 T2 的对象 o2,使得以 T1 定义的所有程序 P 在所有的对象 o1 都替换成 o2 时，程序 P 的行为没有发生变化，那么类型 T2 是类型 T1 的子类型。<br>定义看上去还是比较抽象，可以理解为一个软件实体如果适用一个父类的话， 那一定是适用于其子类，所有引用父类的地方必须能透明地使用其子类的对象，子类对象能够替换父类 对象，而程序逻辑不变。总结一下:子类可以扩展父类的功能，但不能改变父类原有的功能。</p>
<ol>
<li>子类可以实现父类的抽象方法，但不能覆盖父类的非抽象方法。 </li>
<li>子类中可以增加自己特有的方法。</li>
<li>当子类的方法重载父类的方法时，方法的前置条件(即方法的输入/入参)要比父类方法的输入参数更宽松。 </li>
<li>当子类的方法实现父类的方法时(重写/重载或实现抽象方法)，方法的后置条件(即方法的输出/返回值)要比父类更严格或相等。</li>
</ol>
<h2 id="合成复用原则"><a href="#合成复用原则" class="headerlink" title="合成复用原则"></a>合成复用原则</h2><p>合成复用原则(Composite/Aggregate Reuse Principle,CARP)是指尽量使用对象组合(has-a)/ 聚合(contanis-a)，而不是继承关系达到软件复用的目的。可以使系统更加灵活，降低类与类之间的耦 合度，一个类的变化对其他类造成的影响相对较少。</p>
<p>继承我们叫做白箱复用，相当于把所有的实现细节暴露给子类。</p>
<p>组合/聚合也称之为黑箱复用，对类 以外的对象是无法获取到实现细节的。</p>
<p>要根据具体的业务场景来做代码设计，其实也都需要遵循 OOP 模型。</p>
<h1 id="思维导图"><a href="#思维导图" class="headerlink" title="思维导图"></a>思维导图</h1><p><img src="https://tu.dxysun.com/%E8%BD%AF%E4%BB%B6%E8%AE%BE%E8%AE%A1%E5%8E%9F%E5%88%99-20210406233934.png" alt="设计原则"></p>
 
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